1. Field of the Invention
The present invention relates to a coating method for applying a coating solution to a surface of a substrate that is being conveyed.
2. Description of Related Art
In recent years, electricity-powered vehicles that are equipped with a motor as a driving source, such as hybrid vehicles and electrical vehicles, are spreading. The electricity-powered vehicle is equipped with a rechargeable secondary battery. Electrodes that include a strip-shaped metal foil (substrate) and a coating film that is formed on a surface of the substrate by applying a coating solution that contains an active material, a conductive aid, a binder and so on and drying the coating solution are used in a secondary battery. In a secondary battery, a positive-electrode active material that is contained in the coating film on a positive plate and a negative-electrode active material that is contained in the coating film on a negative plate occlude or release ions during charge or discharge. For appropriate occlusion and release of ions, a coating film with an appropriate coating width (length in the direction of the width of the substrate) has to be formed on a surface of respective positive and negative plates.
A coating device is known in which a coating solution is discharged onto a surface of a substrate that is being conveyed by a backup roll from a coating die which is located opposite the backup roll to form a coating. The coating width of the coating film that is formed by the coating die depends on the gap between the discharge port of the coating die and the substrate (coating gap). Thus, Japanese Patent Application Publication No. 2007-258078 (JP 2007-258078 A), for example, discloses a coating method that includes measuring the coating width of the coating solution that is applied by a coating die and adjusting the coating gap by feedback control based on the result of comparison between the coating width measured and a target value. According to this coating method, the coating gap is controlled in a feedback manner based on the coating width measured to adjust the coating width to a target value.
However, the method that is disclosed in JP 2007-258078 A has the following problem. In general, the coating solution that is discharged from a coating die is drawn from a supply tank by a pump or the like and supplied to the coating die through a supply circuit. When the coating gap is changed, the pressure or flow rate of the coating solution that flows through the supply circuit changes accordingly. When the pressure or flow rate of the coating solution that is supplied to the coating die changes, the coating width changes again by virtue of the change in the pressure or flow rate. Thus, even when the coating gap is adjusted based on the coating width measured as described in JP 2007-258078 A, the pressure or flow rate of the coating solution changes by virtue of the change in the coating gap, resulting in the coating width deviating from the target value. In this case, the coating width can be adjusted to a target value by correcting the deviation of the coating width by repetitively controlling the coating gap in a feedback manner. However, it takes as long as approximately 3 to 5 minutes to stabilize the coating width at a target value.
A coating line of this type is usually operated continuously 24 hours a day, and there are a few occasions a year where the production line is stopped and then started again. Thus, it would not matter even if it takes a bit of time to start the coating line again. However, when the coating line is stopped every evening and restarted every morning, or when the coating line is operated continuously 24 hours from Monday to Friday and stopped on Saturday and Sunday, for example, it is strongly desired to stabilize the coating conditions (coating width and pressure or flow rate of the coating solution) quickly at the start of coating. This is because the coating line is operated on a trial basis until the coating conditions are stabilized at the start of the production line. Waste products that are produced during the trial operation increase and the production cost increases unless the coating conditions are stabilized quickly. With the method that is disclosed in JP 2007-258078 A, it takes approximately 3 to 5 minutes and 100 m or more of coated substrate should be disposed of before the coating width is stabilized by feedback control. Doing this every week results in an increase in production cost.